The present invention relates to a method of mixing a pair of components, normally a medicament and a solvent. More particularly this invention concerns such a method and a hypodermic syringe for carrying out the method.
As described in commonly owned U.S. Pat. No. 6,419,656, a medical syringe has a tubular body extending along an axis and having a front end and a rear end, a plunger axially slidable in the body and carrying a stem projecting axially rearward out of the body from the plunger, and a free piston slidable in the body forward of the plunger and subdividing the body forward of the plunger into a front compartment at the front body end and a rear compartment between the plunger and the piston. The body is formed with a bypass passage forward of the piston in a starting position so the front compartment can hold a soluble medicament and the rear compartment can hold its solvent. Structure at the rear body end forms a radially inwardly open angularly limited cutout and at least two axially spaced, angularly offset, and radially outwardly projecting stop bumps on the stem are axially displaceable through the cutout in respective angularly offset positions of the stem. The stop bumps are axially engageable against the structure except when the stem is in the respective angular position. An elastically deformable brake element engaged between the body and the stem for axially slowing axial forward advance of the stem.
Such a system is extremely effective when the medicament is highly soluble in the solvent. When, however, the medicament is less soluble, it is necessary for the user to shake the syringe in order to ensure proper mixing. Such shaking not only is an annoying step, but also risks damage to the syringe. Furthermore it is inexact at best, so that often the patient is injected before all the medicament is fully dissolved, resulting in clogging of the needle or insufficient dosing.
It is therefore an object of the present invention to provide an improved method of mixing a medicament and its solvent.
Another object is the provision of an improved mixing syringe which overcomes the above-given disadvantages, that is which allows the user to fully mix even a hard-to-dissolve medicament.
A syringe holding a solvent and a soluble component has according to the invention a tubular body extending along an axis and having a front end and a rear end and formed therebetween with a bypass, a plunger axially slidable in the body, and a stem projecting axially rearward out of the body from the plunger. A cover fits over the front end of the body and is in accordance with the invention so tight that the front compartment can be pressurized to a superatmospheric pressure without leakage out the front end. A free piston slidable in the body forward of the plunger subdivides the body forward of the plunger into a front compartment at the front body end holding the soluble component and a rear compartment between the plunger and the piston and holding the solvent. Stops are provided for arresting the free piston when it is level with the bypass in a position permitting flow through the bypass between the compartments.
This syringe is used according to the invention by, starting with the plunger and stem in a rearmost end position and the piston at least partially axially rearward of the bypass and separating the solvent from the soluble component, first pressing the plunger and stem axially forward from the rearmost end position into an intermediate position with the plunger rearward of the bypass and thereby pressurizing the rear compartment and forcing the piston forward until the piston in at the bypass and the solvent can flow through the bypass and into the front compartment to mix with the soluble component while gas pressure builds up in the front compartment. When the piston is at the bypass, it is arrested to permit fluid flow in two directions through the bypass past the piston. Then the plunger and stem are released such that gas pressure built up in the front compartment forces the plunger rearward and pumps the solvent and any of the component mixed therewith back through the bypass into the rear compartment.
This pressing and releasing can be done several times to pump the solvent/component mixture back and forth through the bypass between the compartments so as to thoroughly mix them. In this manner even relatively hard-to-dissolve components can be easily and thoroughly dissolved without having to shake the syringe. Normally only a few presses and releases are needed even for the most stubborn substances due to the turbulence of the flow through the bypass which is of restricted flow cross section.
Forward movement of the plunger and stem is stopped in the intermediate position by engagement between formations on the plunger and body. These formations can include short screwthreads on the stem and a short screwthreaded area on the body so that the body must be rotated to advance it forward out of the rearmost position, and then rotated again after mixing is complete to advance it forward from the intermediate position. Instead of screwthreads, bumps can be provided on the stem at angularly and axially offset positions so that the stem has to be twisted to fit the bumps through a notch in the body, such twisting being needed to advance forward from the rear end position and again from the intermediate position.
Once the soluble component in the front compartment is thoroughly mixed with the solvent, according to the invention the front body end is uncovered and fitted with a needle and then the plunger and stem are pushed forward from the intermediate position to press generally all of the solvent/component mixture in the rear compartment into the front compartment until the plunger rearwardly engages the piston. Further forward advance of the plunger, stem, and piston expresses the solvent/component mixture out the front end of the syringe body.